University students' understanding of photosynthesis was examined in a large introductory biosciences class. The focus of this study was to first examine the conceptions of photosynthesis among students in class and then to investigate how a certain type of text could enhance students' understanding of photosynthesis. The study was based on pre- and post-test designs, in which students' prior knowledge of photosynthesis was measured with ten open-ended questions. The test measured the students' systemic and factual levels of understanding. After students read either a text that systematically pointed out how typical misconceptions contradict scientific concepts (a refutational text) or a traditional text (a non-refutational text), the students' knowledge was tested again with the same test instrument. Results showed that students' prior knowledge and understanding of photosynthesis varied considerably, but both texts seemed to support effective learning. The study revealed several common systemic and factual misconceptions students hold concerning photosynthesis. The knowledge of students majoring in biology was higher than that of non-major students, especially concerning the systemic understanding of photosynthesis. The refutational text appeared to help the students who had weak prior knowledge. Thus, science educators should be aware of the heterogeneity of students concerning their levels of prior knowledge and should design instruction accordingly. We suggest that tools, such as refutational texts, can support students at the university level who have weaker prior understanding.